Enhancing the softness of paper products such as tissue and towelling is desirable. Softness is the tactile sensation a user perceives as the user holds, rubs or crumples a particular paper product. This tactile sensation is provided by a combination of several physical properties including the bulk, stiffness and stretchability of the paper. Creping, a process which is well known in the art, is a means of mechanically foreshortening a fibrous structure in the machine direction in order to enhance the softness, bulk and stretchability of the paper. Creping is generally accomplished with a flexible blade, known as a creping blade, which is placed against a drying surface such as a Yankee dryer. The fibrous structure adheres to the Yankee dryer as it contacts the dryer surface. The web travels along the surface of the Yankee dryer until it is removed by the creping blade. The degree to which the fibrous structure adheres to the Yankee dryer prior to creping is a key factor in determining the degree of softness, bulk, and stretchability exhibited by the fibrous structure after creping.
The level of adhesion of the fibrous structure to the Yankee surface is also of importance as it relates to the control of the web in its travel from the creping blade to the reel of the paper machine. Fibrous structures which are insufficiently adhered to the surface of the Yankee dryer are generally difficult to control and often result in quality problems at the reel such as wrinkling, foldovers and weaved edges. Poorly formed paper affects the reliability of the entire papermaking process and subsequent converting operation.
The level of adhesion of the fibrous structure to the Yankee surface is also of importance as it relates to the drying of the web. Higher levels of adhesion permit better heat transfer. This enables the web to dry faster thus allowing the operation to run at higher speeds. Creping aids are generally applied to the surface of the Yankee dryer to further facilitate the adhesion/creping process. It is desirable to deposit the creping aid on the surface of the Yankee dryer.
It has been observed on paper machines utilizing through-air drying, that less creping aid tends to deposit on the dryer surface compared to paper machines utilizing conventional press sections. A through-air dried fibrous structure tends to be transferred to the Yankee dryer at a higher consistency than a fibrous structure which is conventionally wet pressed. Additionally, due to limited knuckle area, a web which is through-air dried, will have a smaller area of contact with the nip of the Yankee dryer pressure roll than a web which is conventionally wet pressed. Thus, through-air dried fibrous structures tend to have poorer adhesion than that of fibrous structures which are conventionally wet pressed.
Prior to the development of creping aids, adhesion of the fibrous structure to the dryer surface was accomplished through the presence of naturally occurring hemicellulose present in the paper fiber. Hemicellulose deposits were observed forming on the surface of the dryer as a result of the evaporation of water from the web. The hemicellulose deposits were found to contain small fiber fragments picked out of the fibrous structure. It was noted that these deposits resulted in the formation of a heavy film on the surface of the Yankee and subsequent poor crepe quality.
It is important that the creping aid allow for a proper balance between adhesion of the fibrous structure to the drying surface and the release of the web at the creping blade. Historically, one of the difficulties encountered with the use of creping aids has been a tendency for the creping aid to form a bond between the web and the drying surface at the point of creping such that the web does not properly release from the drying surface. This results in portions of the web remaining adhered to the surface thus causing defects in the web and/or causing the web to break.
Another common problem associated with the use of creping aids, is an excessive build-up of the creping aid on the drying surface. While some amount of buildup of the creping aid on the surface is essential, excessive buildup can produce streaks which impact the profile of adhesion across the width of the drying surface. This can result in bumps or wrinkles in the finished paper. Quite often, a second blade, known as a cleaning blade, is placed on the drying surface just after the creping blade. The purpose of the cleaning blade is to remove excess creping aid and other residue left behind. Historically, both the creping blade and cleaning blade have had to be frequently changed in order to prevent excessive buildup.
In order to prevent excessive buildup on the drying surface, it is important that the creping aid be rewettable. "Rewettable", as used herein, refers to the ability of the creping aid remaining on the Yankee dryer surface to be activated by the moisture contained in the fibrous structure. A marked increase in tack is indicative of high rewettability. Because through-air drying tends to remove more water than a conventional wet press section, a fibrous structure which is through-air dried will typically be transferred to the Yankee dryer at a higher consistency than a fibrous structure which is conventionally wet pressed. Because a through-air dried web contains less moisture than a similar web which is conventionally wet pressed, there is less moisture available in the through-air dried web to be transferred to the surface of the Yankee dryer. A fibrous structure which has been dried to a relatively higher fiber consistency, such as a through-air dried web, is more difficult to adhere to the drying surface because the web has less water available to rewet the creping aid at the point at which the fibrous structure is transferred to the drying surface.
U.S. Pat. No. 3,926,716, issued to Bates on Dec. 16, 1975 and incorporated herein by reference, teaches a polyvinyl alcohol creping adhesive which forms a film upon application to the surface of the Yankee dryer. It is believed that the polyvinyl alcohol exhibits a rewet mechanism wherein the film already on the surface of the Yankee dryer is rewetted as moisture is released from the fibrous structure at the pressure roll nip of the Yankee dryer.
With the advent of the addition of wet strength resins to the wet end of the paper machine based on polyamide polyamine epichlorohydrin (PAA) wherein the amine is a secondary amine, it was observed that adhesion of the web to the Yankee dryer surface improved. This led to the development of creping aids based on similar PAA secondary amine resin chemistry as that used for wet strength resins. PAA secondary amine based creping aids have had wide acceptance in those paper machine systems using a conventional wet press section. However, in those paper machine systems utilizing through-air drying, creping aids based on PAA secondary amine chemistry have not enjoyed the same success.
This stems from the fact that creping aids based on PAA secondary amine resin chemistry are thermosetting and thus will cure on the heated surface of the Yankee dryer. Creping aids containing thermosetting resins are problematic in that the coating formed by the application of the creping aid to the Yankee dryer on a machine utilizing through-air drying tends to be brittle and exhibits poor adhesion to the dryer surface.
Another problem stems from the wet end addition of wet strength resins which are thermosetting. The thermosetting wet strength resins will actively crosslink with creping aids which contain a secondary amine backbone resulting in the formation of a hard coating on the surface of the Yankee dryer with poor adhesive properties thereby reducing the overall efficacy of the creping aid.
U.S. Pat. No. 4,501,640, issued to Soerens on Feb. 26, 1985, purports to teach a method for creping a cellulosic web wherein an admixture of polyvinyl alcohol and a water-soluble thermosetting cationic polyamide resin are applied to a creping cylinder.
U.S. Pat. No. 5,187,219, issued to Furman, Jr. on Feb. 16, 1993 purports to teach a creping adhesive composition comprising a thermosetting water soluble acrylamide polymer having glyoxal-reacted amide substituents and unsubstituted amide groups and a method of applying the composition.
U.S. Pat. No. 5,494,554, issued to Edwards et al. on Feb. 27, 1996, purports to teach the application of a creping adhesive comprising a thermosetting cationic polyamide resin and a plasticizer to the surface of a dryer.
All of these teachings suffer from a common drawback in that as each of the purported creping aids contains a thermosetting resin. The thermosetting resin will cure on the heated surface of the Yankee dryer forming a brittle coating with poor adhesion characteristics. Additionally, each of the purported creping aids contains secondary amines. The use of creping aids containing secondary amines in conjunction with the addition of a thermosetting wet strength resin will tend to reduce the efficacy of the creping aid.
The objective of the present invention is to provide a method of producing a creped paper product exhibiting overall improved crepe quality.
Another objective of the present invention is to provide a creping aid which exhibits overall improved performance in conjunction with all types of fibrous structures including those through-air dried fibrous structures transferred to the Yankee dryer at higher consistencies.
A further object of this invention is to provide a more efficient method for producing creped paper products wherein the creping aid is not deleteriously impacted by the use of thermosetting wet strength resins.